Thursday, June 27, 2024

A Sorting Adventure in R

As part of an R application on which I have been working, I need to grab some data from an online spreadsheet keyed by a column of labels, add a column with new values, and print out the labels and new values in a two column array so that it matches the spreadsheet (meaning the order is the same as in the spreadsheet). The construction process involves breaking the original data into chunks, filling in the new values in each chunk separately, and gluing the chunks back together. The spreadsheet is sorted by the label column, so to match the original spreadsheet (allowing a user to view them side by side and see the same labels in the same order), I just needed to sort the output data frame by the label column ... or so I thought.

Since I was using the dplyr library already, and dplyr provides an arrange command to sort rows based on one or more columns, I started out with the following simple code (where df is the two column data frame I created):

df |> dplyr::arrange(Label) |> print()

Unfortunately, the result was not sorted in the same order as the spreadsheet. Some of the labels began with numbers (1000) and in one case a number containing a colon (10:20), and arrange listed them in the opposite order from that of the spreadsheet. Some of the name had funky capitalization (say, "BaNaNa"), and arrange treated capital letters as preceding lower case letters. Interestingly, the base R sort function sorted the labels in the same order that the spreadsheet used. More interestingly, the following hack suggested in a response to a question I posted online also matched the spreadsheet:

df |> dplyr::arrange(-dplyr::desc(Label)) |> print()

The desc function tells arrange to use descending order (the default being ascending order). So -desc tells arrange not to use descending order, meaning use ascending order, which is where we started ... and yet it somehow fixes the ordering problem.

The culprit turns out to be the default locale setting for arrange, which is "C". I'm guessing that means the C programming language. I filed a bug report in the dplyr repository on GitHub and was quickly told that the behavior was correct for locale "C" and that the solution to my problem was to feed arrange the optional argument .locale = "en". That did in fact fix things. The code now produces the expected sort order. Meanwhile, my bug report led to a new one about the difference in sort orders between arrange and desc. Depending on how that report is resolved, the -desc trick may stop working in the future.

Wednesday, June 26, 2024

Locating Date Columns in R

I've been working on a script that pulls data from an online spreadsheet (made with Google Sheets) shared by a committee. (Yes, I know the definition of "camel": a horse designed by a committee. The situation is what it is.) Once inhaled, the data resides in a data frame (actually, a tibble, but why quibble). At a certain point the script needs to compute for each row the maximum entry from a collection of date columns, skipping missing values.

Assuming I have the names of the date columns in a variable date_fields and the data in a data frame named df, the computation itself is simple. I use the dplyr library, so the following line of code

df |> rowwise() |> mutate(Latest = max(c_across(all_of(date_fields)), na.rm = TRUE))

produces a copy of the data frame with an extra column "Latest" containing the most recent date from any of the date fields.

That, as it turns out, was the easy part. The hard part was populating the variable date_fields. Ordinarily I would just define it at the front of the scripts, plugging in either the names or the indices of the date columns. The problem is that the spreadsheet "evolves" as committee members other than myself make changes. If they add or delete a column, the indices of the date columns will change, breaking code based on indices. If they rename one of the date columns, that will break code based on a static vector of names. So I decided to scan the spreadsheet after loading it to find the date fields.

It turned out to be harder than it looked. After tripping over various problems, I searched online and found someone who asked a similar question. The best answer pointed to a function in an R library named dataPreparation. I did not want to install another library just to use one function one time, so I futzed around a bit more and came up with the following function, which takes a data frame as input and returns a list of the names of columns that are dates (meaning that if you run the str() command on the data frame, they will be listed as dates). It requires the lubridate library, which I find myself commonly using. There may be more elegant ways to get the job done, but it works.

# INPUT: a tibble
# OUTPUT: a vector containing the names of the columns that contain dates
dateColumns <- function(x) {
  # Get a vector of logical values (true if column is a date) with names.
  temp <- sapply(names(x), function(y) pull(x, y) |> is.Date())
  # Return the column names for which is.Date is true.
  which(temp) |> names()